Methods for continuously forming a plating layer on a surface of a film while carrying the film are described in JP-A-07-22473 and JP-A-2001-192793. In these methods, a conductive surface of a non-metallic film or a metallic film is brought into contact with a cathode roll, and the film is carried and passed through a plating bath arranged in the upstream and/or downstream side of the cathode roll and accommodated with a plating solution and an anode, for forming a plating layer on the conductive surface in the plating bath. In these methods, plural units, each consisting of a cathode roll and a plating bath, are installed, and the film is carried and passed through the plural plating units one after another, to form a plating layer having a desired thickness on the conductive surface of the film.
Substrates for flexible circuits are used in electronic apparatuses, electronic parts, semiconductor packages, etc. As one of the substrates, a wiring board consisting of a polyimide film or polyester film and a copper foil is used. As the wiring boards, there are a board usually called “a three-layer type” in which a copper foil is stuck to a film through an adhesive and a board usually called “a two-layer type” in which a metallic layer is formed on a film by means of plating or the like without using an adhesive. Recently the wiring of circuits is formed at a finer pitch, and among these board types, the latter two-layer type attracts more attention.
In a three-layer board for printed circuit, an epoxy-based resin or acrylic resin is used as the adhesive. This board has a disadvantage that the electric properties are deteriorated due to impurity ions contained in the adhesive. The heat resistance temperature of the adhesive of the board is from 100° C. to 150° C. and therefore, in the case where a polyimide film is used as the base film, the board has a disadvantage that the high heat resistance of the film of 300° C. or higher cannot be sufficiently utilized. As a result, in the wire bonding to an IC chip in need of high temperature, the heating temperature used must be lowered.
In a three-layer board for printed circuit, the general thickness of the copper foil is 18 μm or 35 μm. Therefore, in the case where patterning is performed at a wire pitch of 80 μm (the copper wire width: 40 μm and the inter-wire gap: 40 μm) or less, copper is so thick as to remarkably lower the etching rate, and in addition, the circuit width on the surface side of the copper foil becomes very different from the circuit width of the adhesive side. Otherwise, there occurs a phenomenon that etching makes the entire circuit width very thin, not allowing a desired circuit pattern to be obtained.
To solve the above-mentioned problems in the three-layer type, a two-layer board for printed circuit free from the use of an adhesive is proposed. A two-layer board can be produced by vapor-depositing any of various metals on a film by any of various vapor deposition methods such as vacuum evaporation, sputtering or any of various ion plating methods (including PVD method, CVD method of vaporizing chemicals including a metal for deposition), to form a conductive surface, or plating a film with any of various metals by an electroless plating method to form a conductive surface, and subsequently electrolytically plating the conductive surface with copper.
In a two-layer board, the thickness of the copper layer can be changed freely by means of electrolytic copper plating. For example, if a copper layer having a thickness of 8 μm is obtained, a circuit pattern at a wire pitch of 60 μm can be simply produced, and the heat resistance of any of various base films can be sufficiently utilized. Based on such situation, the demand for plated films is growing.
However, when a continuously carried film is plated, the magnitude of the tension acting on the film in the carrying process cannot be so large in view of the stiffness of the film.
Furthermore, unless there is certain slipperiness between the carried film and the cathode roll, the carried state of the film is disturbed by the cathode roll, and the carrying tension is unbalanced at plural portions in the width direction of the film. The unbalance causes the carried film to be wrinkled and creased, making the carried state unstable.
With regard to this problem, in the conventional apparatus, the running film that accompanies the plating solution taken from the plating bath reaches the cathode roll, to form a liquid layer on the cathode roll, and this liquid layer solves the problem. The liquid layer gives some slipperiness between the film and the cathode roll. The slipperiness prevents that the carried state of the film becomes unstable at plural portions in the width direction of the film.
Furthermore, as described in JP-A-2001-192793, if the carried film is a metallic foil such as a copper foil, the carrying tension of the film can be made large, and the surface resistance value can also be kept small, allowing perfect conduction with the cathode roll to be achieved, without causing any problem.
However, as described in JP-A-07-22473, if it is attempted to carry a polyimide film having a thickness of 50 μm, the film may be fractured in view of the Young's modulus and strength of the film, etc. Furthermore, an internal stress may work in the formed plating layer. Therefore, a large tension cannot be applied to the film, and employed is a method of forming a plating layer while the carried state of the film is balanced under a relatively low film tension. That is, a liquid layer containing a plating solution is interposed between the cathode roll and the film, for causing adequate slipperiness between the cathode roll and the film, to stabilize the carrying of the film.
However, even in this method, if an electric current is fed from the cathode roll for forming a plating layer, it often occurs that the metal intended to constitute the plating layer is precipitated and deposited on the cathode roll.
There occurs a phenomenon in which the metal deposited on the cathode roll is brought into the plating bath by the film and acts as nuclei in the plating bath, to form abnormal projections (projection defects) on the conductive surface or the plating layer surface due to electric field concentration. There also occurs a phenomenon in which the metal deposited on the cathode roll forms depressions in the film or flaws the conductive surface or plating layer surface. The thickness of the plating layer formed further subsequently is not enough to flatten the depressions or the flawed portions. As a result, the surface of the plated film produced has depression defects. Moreover, the patterns of the metal deposited on the cathode roll are transferred to the conductive surface of the film, to cause a problem of degrading the surface appearance quality.
The depression and projection defects formed on the surface of the plated film may cause such non-conformances as wire breaking in the etching step for forming circuit wiring or in the step of bonding IC chips and the like in the circuit mounting process, to cause a problem that the quality of the circuit cannot be guaranteed.
The method for inhibiting the precipitation of the metal intended to constitute the plating layer on the cathode rolls causing such problems is not yet found. At present, after the apparatus has been operated for a certain time, the production is stopped, and the metal precipitated and deposited on the cathode rolls is scraped off. Then, the operation of the apparatus is resumed. The metal removal from the cathode rolls remarkably lowers the productivity of the plated film.
On the other hand, the conventional cathode rolls are formed of an iron-based material. Furthermore, the plating solution is based on sulfuric acid, and it often contains hydrochloric acid. Therefore, the cathode rolls have a problem that the selection of the material for corrosion prevention is difficult. In this situation, SUS316 has been suitably used as a material resistant against the plating solution. However, even the SUS316 has a problem that intergranular corrosion is caused after it has been used for a certain time.
The cathode rolls function also as rolls for carrying the film. The cathode rolls have a problem that they are rubbed by the running film and are gradually flawed. Therefore, if the cathode rolls are used for production for a certain time, they are flawed to increase the frictional force with the film, causing such a state that the film is gripped. This state causes the film to be tensioned and deflected and further causes it to sway irregularly in the horizontal direction, thereby wrinkling it during the carrying process. In the worst case, the running film is creased.
Furthermore, as the case may be, it can happen that the metal intended to constitute the plating layer, for example, copper is precipitated and deposited on the cathode rolls. If the deposit, for example, copper is scraped off for being removed, using a sponge containing an abrasive, the SUS316 used as the material constituting the cathode rolls is often flawed.
The flaw patterns are transferred to the plating layer, for example, copper layer of the plated film produced. The plated film having the transferred flaws has surface appearance quality defects called hair lines. The hair lines are depression flaws, and cause wire breaking in the etching step for forming circuit wiring and in the step of bonding IC chips in the circuit mounting process. As a result, the quality of the produced circuit may not be able to be guaranteed.
Moreover, the cathode rolls used for plating are heavily consumed due to flawing. In the case where the cathode rolls are used for continuous production, they must be exchanged for being reground after use for two weeks or about one month. This greatly increases the maintenance work and cost of the apparatus, to lower productivity, hence to raise the production cost.
To solve this problem, JP-3135176 proposes a method in which a plating surface, for example, copper surface in a product of a flexible circuit board is not brought into contact with a cathode roll. This is a so-called contact-less carrying method and at now it is used for manufacturing a plated film. An outline of this method is shown with FIG. 17. In FIG. 17, carrier rolls 52 and 53 have large-diameter discs 52a, 52b, and 53a, 53b at their both ends respectively. A film 50 is carried by being guided both edge portions 51a, 51b of the film 50 with these discs. In FIG. 17, symbol W denotes the overall width of the film 50, and symbol Wa denotes the width of the non-contact portion. When the film 50 is carried, a liquid is made to flow from the inside of the carrier roll 53 toward the outside, for letting a force for biasing the film 50 toward outside acts on the film 50. The liquid is fed from a liquid source 55 through a flow rate control unit 56, being injected from the respective nozzle holes 58 of a nozzle pipe 57.
However, if the film 50 is carried by this method, the liquid giving a biasing force from the inside of the roll becomes unstable, and the film 50 deviates from the large-diameter portions (disc portions) of the carrier roll, not allowing continuous production. Furthermore, in the case where the carrier roll 53 is a cathode roll, when the power is supplied from both the ends, the power supply area is small, and the power supply is likely to be unstable. As a result, the thickness of the formed plating layer becomes irregular. The cathode roll for plating is heavily consumed, and in the case of continuous production, the cathode roll for plating must be exchanged for being reground after use of two weeks to about one month. This greatly increases the maintenance work and cost of the apparatus, to lower the productivity and to raise the production cost.
Electric and electronic apparatuses are being substituted by IC version, and become more highly dense and more highly integrated at a rapid pace. In this connection, the wire pitch of patterns of flexible printed circuit boards become finer from a pitch ranging from 150 to 200 μm to a pitch ranging from 80 to 150 μm, and presently there is a demand for producing patterns at a wire pitch ranging from 30 to 80 μm. In future, a demand for producing patterns at a wire pitch of less than 80 μm is expected to arise.
An object of the invention is to provide a method for producing a plated film that solves the above-mentioned problems of the prior art and can meet the above-mentioned demand.
Another object of the invention is to provide a cathode roll usable in the method for producing a plated film that can solve the above-mentioned problems of the prior art and can meet the above-mentioned demand.